This invention relates to an improved method of refining potassium chloride brine by multiple stage evaporation and multiple stage crystallization and particularly relates to passing in heat exchange relationship streams in multiple effect evaporation with streams in multiple stage crystallization.
Brine containing potassium chloride typically contains also sodium chloride in admixtures therewith as well as a small amount of chlorides and sulfates of calcium and magnesium. Those brines can arise as a result of an aqueous solvent coming in contact with an ore containing potassium chloride, sodium chloride and the aforementioned impurities, as the result of a lake or other large natural body of water which has been adjacent to the ore, or as solutions of subterranean deposits of the ore which has been either shaft mined or solution mined.
Most often these brines are concentrated with respect to potassium chloride by evaporation of water therefrom at elevated temperatures and selective precipitation of the sodium chloride and other salt impurities therein. The aforesaid method of concentration can be utilized owing to the solubility of sodium chloride and other salt impurities remaining virtually constant or decreasing with increasing temperatures and owing to the solubility of potassium chloride increasing with increasing temperatures. Evaporation can be carried out at progressively higher temperatures, such as by multiple effect evaporation, whereby the brine is brought near or to saturation with respect to potassium chloride in each effect so that in the final and hottest effect the brine is highly concentrated.
An efficient method of operating multiple effect evaporators is by backward feeding. In this operation, the raw brine feed enters the last (coldest) effect. The mother liquor effluent from the last effect becomes the feed to the next to the last effect, and so on until a concentrated mother liquor is withdrawn from the first effect. Steam from an external source is condensed in the heating element of the first effect. Steam generated in the first effect is condensed in the heating element of the second effect and so on to the last effect. This gives rise to an efficacious method of evaporating a relatively cool feed, such as potassium chloride brines from the above described sources. Very high temperatures can be attained in the evaporators when they are operated well above atmospheric pressure, thereby highly concentrating the brine.
After the evaporation stage, the concentrated brine is cooled to precipitate of crystalline potassium chloride. Methods such as multiple stage evaporative crystallization are used to expediently precipitate potassium chloride from the brine. In that type of crystallization, the latent heat of vaporization is effectively provided by the sensible heat of the brine, thereby cooling the brine. Most often, the evaporative crystallizers are operated under progressively increased vacuum for each successive stage utilizing barometric condensers, which are aided by steam ejector jets in the cooler stages. The brine is passed through the crystallizers in the direction of increased vacuum. Hence, the hottest crystallizer is called the first stage and the coolest the last stage. Mother liquor effluent from the coolest stage, having been depleted of potassium chloride, can be recycled for further concentration.
The above described process is effective in producing potassium chloride crystals from its brines. However, a great amount of energy is consumed in supplying heat to evaporate water from the brine. Moreover, availability of steam for ejector jets and cooling water for barometric condensers used with crystallizers can be limited. So, it is always a desideratum that the amount of energy consumed be reduced and that the requirement of cooling water lessened even if it is only for the purpose of reducing heat pollution of natural bodies of water.